e. References. The addresses for ordering the latest revision of advisory circulars and other referenced documents listed below can be found in the Appendix to this AC.
AC 20-34D, Prevention of Retractable Landing Gear Failures.
AC 23.729-1, Landing Gear Doors and Retraction Mechanism. (For information only).
AC 25-7A, Flight Test Guide for Transport Category Airplanes.
AC 43.13-1A, Acceptable Methods, Techniques and Practices - Aircraft Inspection and Repair.
AC 25.963-1, Fuel Tank Access Covers.
SAE AIR-4566 - Crashworthiness Landing Gear Design.
SAE ARP-1311A - Landing Gear- Aircraft.
f. Definitions. For definitions of VS, VS1, and VC, see 14 CFR part 1, section 1.2, titled Abbreviations and symbols.
12. SECTION 25.731 - WHEELS.
a. Rule Text.
(a) Each main and nose wheel must be approved.
(b) The maximum static load rating of each wheel may not be less than the corresponding static ground reaction with-
(1) Design maximum weight; and
(2) Critical center of gravity.
(c) The maximum limit load rating of each wheel must equal or exceed the maximum radial limit load determined under the applicable ground load requirements of this part.
[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-72, 55 FR 29777, Jul. 20, 1990]
b. Intent of Rule. This rule requires use of approved wheels, either approved under an applicable Technical Standard Order (TSO) e.g., TSO-C26c, or approved as part of the type design for the airplane. Wheels must satisfy both a design static (1g) load and design limit landing or taxiing load determined under the applicable ground load requirements (§§ 25.471 through 25.511). Standards for a tire installed on a wheel are contained in § 25.733. Standards for a brake installed on a wheel are contained in § 25.735. A TSO approval is not an approval to install wheels on the airplane. If the airframe manufacturer decides to install equipment approved under a TSO, they must conduct the applicable airplane certification tests and obtain FAA approval for the installation.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.335(a) and 4b.335(b) of the CAR became §§ 25.731(a) and 25.335(b) of 14 CFR for wheels.
(1) Amendment 25-72 (July 20, 1990) amended the rule to become compatible with § 25.25 (which had been amended). This amendment provides for weights that are in excess of takeoff weight, such as ramp weights, provided that the compliance with applicable structural requirements, including wheel strength, is demonstrated at the higher weights.
(2) Harmonization. This regulation is the subject of a Federal Aviation Regulation/Joint Aviation Requirements (FAR/JAR) harmonization effort under the Aviation Rulemaking Advisory Committee (ARAC). The ARAC working group is revising §§ 25.731 and 25.735, and is developing a new advisory circular AC 25.735-1, and a new TSO-C135 for transport category airplanes, replacing the applicable parts of the existing TSO-C26c.
d. Policy/Compliance Methods. For guidance on compliance, the following material is extracted from Advisory Circulars (AC) 27-1, Certification of Normal Category Rotorcraft, and AC 29-2B, Certification of Transport Category Rotorcraft. It is applicable to certification of transport category airplanes. While not specifically prepared for transport category airplanes, this policy material has been used for demonstrating compliance with § 25.731. For additional guidance, see AC 25-7A, Flight Test Guide for Transport Category Airplanes, chapter 4, section 4, par 53, issued March 31, 1998.
(1) The structural design loads data shall contain both a static load and a landing and taxiing load for each wheel. These loads are determined by virtue of compliance with the standards of §§ 25.731(b) and 25.731(c). The ratings of the wheel shall not be exceeded.
TSO-C26c contains minimum performance standards for TSO approval of aircraft wheels and wheel-brake assemblies. Ratings are assigned in accordance with this performance standard.
(2) If a wheel selected for an aircraft design has TSO-C26c approval, the wheel manufacturer will supply the rating to the aircraft manufacturer. Each wheel shall be marked as prescribed which includes a listing of the TSO number. Even though a wheel is TSO approved, the application on the aircraft (loads imposed on the wheel) requires proof that the rating is not exceeded. Wheel rating must not be less than airplane maximum radial load limits.
(3) If a wheel selected for an aircraft design is not approved under TSO-C26, the necessary data, both detail design and assembly drawings and qualification tests and test report data, will be required to comply with the standards contained in part 25. Design control and inspections will be accomplished as a part of the aircraft type design. Structural substantiation and any appropriate qualification tests shall be accomplished. See §§ 25.471 through 25.497 for the ground load conditions.
(4) The Tire and Rim Association Inc., issues a yearbook listing aircraft tire and rim sizes and ratings. The dimensions and contours for aircraft wheel rims are contained in the yearbook.
e. References. The addresses for ordering the latest revision of advisory circulars, technical standard orders, and other referenced documents listed below can be found in the Appendix to this AC.
Section 25.731 Wheels and § 25.735 Brakes, part 25 of 14 CFR.
AC 25-7A, Flight Test Guide for Transport Category Airplanes.
AC 27-1, Certification of Normal Category Rotorcraft.
AC 29-2A, Certification of Transport Category Rotorcraft.
AC 43.13-1A, Acceptable Methods, Techniques and Practices-Aircraft Inspection and Repair, Chap. 8, L. G. Equipment.
TSO-C26, was an acceptable standard to Civil Aeronautics Administrator, but was not part of the regulations.
TSO-C26a, was a part of the regulations, under part 514 of the Regulations of the Administrator, as § 514.72.
TSO-C26b, was part of the regulations as § 37.172 of 14 CFR.
TSO-C26c, was part of the regulations as § 37.172 of 14 CFR.
TSO-C26c with Addendum I. The TSO was removed from regulations (§ 37.172, Amendment 25-52, 1980) and became a voluntary standard.
SAE AIR-811B, Disposition of Wheels which have been Overheated.
SAE ARP-1322, Overpressurization Release Devices.
SAE ARP-1786, Wheel Roll on Rim Criteria for Aircraft Application.
SAE AS-707B, Thermal Sensitive Inflation Pressure Release Devices for Tubeless Aircraft Wheels.
13. SECTION 25.733 - TIRES.
a. Rule Text.
(a) When a landing gear axle is fitted with a single wheel and tire assembly, the wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions and with a load rating approved by the Administrator that is not exceeded under-
(1) The loads on the main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, and
(2) The loads corresponding to the ground reactions in paragraph (b) of this section, on the nose wheel tire, except as provided in paragraphs (b)(2) and (b)(3) of this section.
(b) The applicable ground reactions for nose wheel tires are as follows:
(1) The static ground reaction for the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position with a force of 1.0g acting downward at the center of gravity.
This load may not exceed the load rating of the tire.
(2) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum landing weight) and center of gravity position combined with forces of 1.0g downward and 0.31g forward acting at the center of gravity. The reactions in this case must be distributed to the nose and main 19
wheels by the principles of statics with a drag reaction equal to 0.31 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(3) The ground reaction of the tire corresponding to the most critical combination of airplane weight (up to maximum ramp weight) and center of gravity position combined with forces of 1.0g downward and 0.20g forward acting at the center of gravity. The reactions in this case must be distributed to the nose and main wheels by the principles of statics with a drag reaction equal to 0.20 times the vertical load at each wheel with brakes capable of producing this ground reaction. This nose tire load may not exceed 1.5 times the load rating of the tire.
(c) When a landing gear axle is fitted with more than one wheel and tire assembly, such as dual or dual-tandem, each wheel must be fitted with a suitable tire of proper fit with a speed rating approved by the Administrator that is not exceeded under critical conditions, and with a load rating approved by the Administrator that is not exceeded by-
(1) The loads on each main wheel tire, corresponding to the most critical combination of airplane weight (up to maximum weight) and center of gravity position, when multiplied by a factor of 1.07; and
(2) Loads specified in paragraphs (a)(2), (b)(1), (b)(2), and (b)(3) of this section on each nose wheel tire.
(d) Each tire installed on a retractable landing gear system must, at the maximum size of the tire type expected in service, have a clearance to surrounding structure and systems that is adequate to prevent unintended contact between the tire and any part of the structure or systems.
(e) For an airplane with a maximum certificated takeoff weight of more than 75,000 pounds, tires mounted on braked wheels must be inflated with dry nitrogen or other gases shown to be inert so that the gas mixture in the tire does not contain oxygen in excess of 5 percent by volume, unless it can be shown that the tire liner material will not produce a volatile gas when heated or that means are provided to prevent tire temperatures from reaching unsafe levels.
[Amdt. 25-23, 35 FR 5676, Apr. 8, 1970; Amdt. 25-38, 41 FR 55467, Dec. 20, 1976; Amdt. 25-49, 44 FR 68745, Nov. 29, 1979; Amdt. 25-72, 55 FR 29777, Jul 20, 1990, as amended by Amdt 25-78, Mar 29, 1993]
b. Intent of Rule. This rule specifies type certification requirements for both design and performance of tires used on transport category airplanes. The tire must be of proper fit and have approved speed and load ratings for a particular airplane application. The maximum static ground reaction for the condition specified must not exceed the maximum static load rating of each tire. Retractable gear system tires must have adequate clearance from surrounding structure and systems. The tire inflation medium is to be an inert gas to avoid explosions. Tires installed on landing gear axles with multi-wheels (main wheel tires only), must have a 7% load margin included in their rating. Tire performance standards are contained in Technical Standard Order (TSO) TSO-C62. A TSO approval is not approval to install tires on the airplane. The airframe manufacturer/user must conduct the applicable airplane certification tests and receive FAA approval for installation.
c. Background. Effective February 1, 1965, part 25 was added to Title 14, Code of Federal Regulations (14 CFR) to replace part 4b of the Civil Air Regulations (CAR). Sections 4b.336(a) and 4b.336(b) of the CAR became §§ 25.733(a) and 25.733(b) of 14 CFR for tires.
(1) Amendment 25-23 (April 8, 1970) added tire speed rating as a limitation governing the acceptance of tires for use on particular airplanes, identified the FAA Administrator to approve load and speed ratings in lieu of The Tire and Rim Association, and added the term "suitable" to "the tire of proper fit."
(2) Amendment 25-38 (December 20, 1976) added § 25.733(c) to require that for retractable landing gear, the design must account for tire production tolerances and size increases expected in service. The tire should have adequate clearance to prevent jamming or interference of landing gear mechanisms and equipment/structure.
(3) Amendment 25-49 (November 29, 1979) improved minimum performance standards applicable to landing gear main and nose wheel tires (by revising § 37.167/TSO-C62b to TSO-C62c) and added more comprehensive design standards covering tire loads and speed ratings (§ 25.733). This amendment also specified a date (December 31, 1979) after which tire manufacturers could no longer identify their tires as approved under earlier standards (TSO-C62, C62a, C62b).
(a) This amendment evolved as follows: The minimum performance standards for tires were set forth in the technical standard order (§ 37.167 "Aircraft Tires,” TSO-C62b) which was unchanged since it became effective in 1962. A series of accidents and incidents involving large commercial jet airplanes, particularly wide body types, in the mid 1970's involved failures of tires, wheels, brakes, and antiskid devices. The FAA intensified its surveillance efforts with respect to aircraft tires and began an analysis of the failures and potential corrective actions.
This led to joint FAA-Industry meetings in 1976 and 1977 that resulted in a proposed set of revised and updated standards (Notice 79-7) to reflect the latest technology and to meet operating conditions.
(b) This amendment was a response to recommendations from the National Transportation Safety Board (NTSB). The NTSB issued a series of recommendations and advisory actions affecting tires in the areas of design standards, qualification testing, quality control during manufacture, and operational limits.
(4) Amendment 25-52 (June 2, 1980) revoked part 37, § 37.167 Aircraft Tires - TSO- C62c. Subpart A of 14 CFR part 37 became Subpart O of 14 CFR part 21. Subpart B of 14 CFR part 37 was eliminated, and the technical performance standards were made available at FAA Headquarters.
(5) Amendment 25-72 (July 20, 1990) deleted the requirement to consider the effects of inertia, from § 25.733(a)(1) because such effects were determined to be negligible for constant speed taxi and takeoff conditions. In addition, § 25.733(c)(1) changed from a reference to § 25.733(a)(1) to a direct statement of the loading conditions for the sake of clarity.
(6) Amendment 25-78 (March 29, 1993) added paragraph § 25.733(e) to require that for airplanes with a maximum certified takeoff weight of more than 75,000 pounds, the tires mounted on braked wheels be inflated with dry nitrogen or other inert gases so that the tire does not contain oxygen in excess of 5 percent by volume to prevent tire explosions. The 75,000 pounds weight limit was based on a review of the service difficulty reports indicating that tire explosions, as opposed to tire bursts, tend to occur on the larger, heavier airplanes. The 5 percent by volume limit for oxygen content was based on a series of laboratory tests indicating that an abrupt auto-ignition could occur for oxygen concentrations of 10 percent or more.
(a) There had been several cases where tire explosions had occurred in transport category operations. A tire explosion differs from a tire burst, which can occur when an overheated or over inflated tire fails and releases the high pressure air contained therein.
Protection against tire burst is required under § 25.729(f). A tire explosion is the result of a chemical reaction occurring when gases released from overheated tire material mix with oxygen in the inflation air and ignite. In 1987, the FAA issued an airworthiness directive (AD 87-08-09) requiring use of nitrogen for tire inflation to ensure that the tires on braked wheels of airplanes do not contain more than 5 percent oxygen. Amendment 25-78 was intended to accomplish the same purpose for new airplanes.
d. Policy/Compliance Methods. For guidance on compliance with this requirement, refer to the preamble of this rule and the following information.
(1) Approval of Retreaded Tires by Similarity. The following is extracted from an FAA memorandum dated March 1, 1995, that addresses whether “qualification by similarity” basis could be used for approving the production of retreaded aircraft tires, using a modified tread compound, in various sizes.
(a) Background. A tire manufacturer had qualified several retreaded tires of different sizes in accordance with the stipulations of advisory circular (AC) 145-4, Inspection, Retread, Repair, and Alteration of Aircraft Tires (dynamometer testing). The manufacturer wished to produce other retreaded tires of different sizes and ratings, based on similarity. In the past, every tire size had to be tested for such an approval.
(b) Supportive Information. The following supportive information was provided:
1 Nature of the Change The formulation for the tread compound used to retread commercial aircraft tires had been modified as follows:
(aa) Polymer: A proprietary description of the change.
(bb) Filler: A proprietary description of the change.
2 Test Results. The change in tread compound had been proven by dynamic qualification testing on a sample of tire sizes. The various tire sizes were representative of the tire sizes listed for production approval in the applicant's letter. Qualification Test Reports (QTR’s) were submitted to the FAA. The QTR’s were approved by the cognizant Flight Standards District Office.
3 Physical Properties of the Tread Material. The physical properties measured in a laboratory showed that the new compound had a specific percent lower hysteresis value, an indicator of heat generation. This characteristic provided an indication of tread durability, and related to the qualification of tires that had been retreaded using the new compound. Also, this lower modulus and a lower hardness of the new compound indicated a better resistance to scuffing and tearing actions to which the tire is subjected in service.
4 Static Test Results per AC 145-4. Because the tire design remained the same, all static measurements (overall diameter, section width, static-loaded radius, skid depth, and static unbalance) were not affected by this change. Tread weight remained within the retread process capability.
5 In-Service Evaluation. Tires retreaded with this new material had been evaluated in service with several different airlines. No problems had been reported by the operators and no tires had been prematurely removed from service due to tread-related conditions, such as chunking or tread separation. Field inspection and examination of returned worn casings, performed by qualified personnel, had indicated good tread integrity and wear characteristics. Based on cycle data gathered (number of landings per retread), the length of the maintenance interval remained within a reasonable percentage of the baseline tire.
6 Rationale Summary. The tire sizes selected to demonstrate the continuation of performance were representative of the tire sizes for which production approval was requested. At least one tire size was qualified in each of the tire design types presently in service. For each tire application (nose and main landing gear), at least one tire was dynamically qualified. With one tire size, the highest speed rating in the list of proposed tire sizes was successfully qualified. With another tire size, the tested sample included the highest rated pressure tire. With yet another tire size, the largest skid depth in the list of proposed sizes was 23
dynamically tested. Within their own class of tires, these were, historically, the most difficult sizes to meet the qualifications of the AC 145-4 requirements. They had often been used in the past to prove process or material changes.
7 In view of the substantiating factors detailed above, it was requested that all of the additional tire sizes listed in their letter be authorized for production on the basis of “qualification by similarity.”
(c) Approval. The FAA carefully considered the information provided above, and determined that there was sufficient merit to authorize production of the additional tire sizes for retreading, based on ”qualification by similarity.”
(2) Approval of Nonretreadable Tires - Qualification Testing. The following is extracted from an FAA memorandum dated August 18, 1988, which addresses the definition and application of "major and minor" changes as they relate to products with a TSO versus aircraft installation approval.
(a) TSO tire dynamometer testing, alone, is not considered acceptable for installation approval except for the most straightforward changes. This memorandum also addresses a potentially unsafe condition that may have resulted from retreading tires that were designed and approved as nonretreadable.
1 Based on evidence presented, including the adverse service history, it appears that the development of the lightweight and nonretreadable tires should have been classified as a "major change," which would have necessitated flight testing on the candidate airplane(s) prior to approval.
(b) It was noted that the TSO was granted based on dynamometer testing, and that the tire was presumably shown to meet the required load-speed-time curve, and other airplane weight and load parameters. However, FAA policy has been that dynamometer testing is not a suitable substitute for airplane tests for any but the most straightforward changes.
(c) The memorandum recommended that a suitable test program be implemented as soon as possible that would demonstrate that these tires are appropriate for their intended application. The test program that was established in January 1984, for the introduction of radial tires, presented later in paragraph (4)(b), is also appropriate in this case since the effect of these tires on airplane performance, landing gear dynamics and antiskid operation, wheel/fuse plug integrity, brake energy absorption, and wheel-well tire burst, is unknown. These lower weight, and therefore lower strength, tires may have a profound effect on the airplane performance, the wheel/brake integrity, or the life of the tires. The documentation that accompanies nonretreadable tires, once they are approved, must clearly show that the tires are not to be retreaded.
(d) There is concern that there have been many changes to wheels, tires, and brakes that have been considered to be "product improvements," and approved under the TSO 24
system as "minor changes," that should have been demonstrated on the airplane prior to their approval.
(3) Approval Method for Substitute Tires. The following is extracted from an FAA memorandum dated April 14, 1988, which addresses the certification considerations that should be taken into account when the installation approval of a new tire on a type certificated aircraft is requested.
(a) A substitute tire must be shown to be adequate for its intended use. In some cases, the proposed tire will be listed in the airframe manufacturer's specification documents, and can be installed without further investigation. The airframe manufacturer should be contacted to determine if data exist regarding the acceptability of the proposed tire to be substituted on the airplane.
(b) If the replacement tires are of the same basic type and construction as the currently approved tires, but they have not been previously approved on the airplane, the following items should be considered. This list is not meant to imply that flight tests would be mandatory; rather, that the items should be discussed with the applicant and any discrepancy or concerns addressed.
1 The TSO load-speed-time curves should be compared to the airplane envelope to establish the suitability of the proposed tire for installation on the airplane. If the tire is not TSO authorized, it would be the applicant's responsibility to demonstrate that the tire meets all TSO requirements.
2 The physical dimensions of the mounted tire should allow adequate clearance to the wheel well and its contents, landing gear structure, gear doors, etc.
Consideration should be given to new tires as well as retreads and possible growth effects of retreading.
3 If the weight of the proposed tire is significantly different from the original, or from other tires approved on the airplane, landing gear retraction times or emergency gear extension operation may be affected. A flight test should be conducted if there is a concern that any weight difference would be large enough to affect gear actuation times.
4 The spring rates for the proposed tire should be compared to other tires approved for the specific airplane at the expected load and deflection, to determine any adverse effect on the loads on the landing gear or any adverse dynamic coupling between the landing gear and tires.
5 The tire rolling radius, tire pressure, tire deflection, and tire footprint should be compared to other tires approved on the airplane, to determine the effect on water spray patterns or tire heating. If the tire pressure is adjusted to obtain a certain deflection, the effect on tire load carrying capability should be investigated.
6 Uninstrumented functional landings should be conducted. If antiskid is installed, these functional landings should be conducted on both wet and dry runways to determine if any adverse effects on the system have been introduced by the proposed tire.
7 If comparison of the proposed tire to other tires approved on the airplane reveals any significant differences in the factors noted above, or if there is evidence of significantly increased or decreased tire rolling resistance or tire µ, performance landings should be conducted. Increased tire rolling resistance could impact takeoff performance, decreased rolling resistance could affect fuse plug integrity, and a different tire µ could affect stopping performance.
(4) Replacement of Bias Ply Tires with Radial Ply Tires. The following is extracted from an FAA memorandum dated February 7, 1984.
(a) This memorandum provided certification criteria for installation of radial tires on type certificated aircraft which only had bias tire approval. The radial tire had obtained TSO approval and this memorandum outlined the installation considerations.
(b) Certification Program for Replacing Bias Tires with Radial Tires, part 25 Transports; All weight Categories, dated Jan. 12, 1984. A bias ply to radial ply tire change is not considered minor. The following certification program is designed for a TSO approved radial tire that is to replace a bias tire of equivalent rating.
1 The tire must be TSO-C62c approved, or equivalent as stated in § 25.733(c), and rated for the speed and gross weight under consideration. The radial tire must have been tested to the equivalent of the TSO, i.e., loads, speeds, and energies, etc.
2 The vertical spring rate for the radial tire should be determined and compared to the original bias tire in order to assess the difference in energy absorption characteristics. The airplane vertical load factors should not exceed the original design values with the radial tire installed. Also, spin-up and spring-back loads should be evaluated if there are significant differences in the weight and moments of inertia of the two tire and wheel assemblies.
(For guidance on compliance refer to §§ 25.479, 25.723, and 25.725.)
3 The protection of the equipment in the wheel wells should be reassessed as the radial tire will have different failure burst characteristics than the bias tire. This assessment should cover tire air blast, as well as loose tread. (For guidance on compliance refer to § 25.729(f).)
4 Emergency gear extension should be reviewed and, if necessary, reassessed if tire size or weight changes are involved. (For guidance on compliance refer to § 25.729(c).)
5 A maximum Rejected Takeoff (RTO) energy stop using the 3 mile taxi, 3 full stops criteria of AC 25-7A, Flight Test Guide for Transport Category Airplanes, chapter 4, section 4, paragraph 55, should be conducted.
(aa) The tire pressure is to be set to the highest value appropriate for the maximum takeoff weight for which approval is being sought. Pressure should be set before taxi and with cold tires. This test is specified to confirm (at maximum takeoff weight) acceptable tire structural integrity, tire anti-skid dynamics, airplane braking performance, effectiveness of the fuse plugs, and to assess the directional control compatibility of the airplane if the fuse plugs melt late in the RTO run. In addition, the fire limitations of AC 25-7A, paragraph 55c(ii), or latest revision, are to be observed.
6 A sufficient number of RTO's and landings should be conducted to substantiate that the aircraft performance with radial tires is as good as, or better than, originally demonstrated with bias tires. Tests should be conducted in accordance with AC 25-7A, paragraph (2)(ii)(c), or latest revision.
7 The fuse plug integrity no-melt tests should be conducted in accordance with AC 25-7A, paragraph 55c(7), unless the applicant demonstrates the energies absorbed by the brake with a radial tire are no greater than the energies absorbed in the same brake when using the bias tire when comparing equivalent flywheel energies on the dynamometer. The fuse plug integrity test should also be run if the brake or tire temperatures of the radial tire/wheel/brake assembly are greater than the brake or tire temperatures of the bias tire/wheel/brake assembly when comparing equivalent flywheel energies on the dynamometer.
8 A sufficient number of flight tests should be conducted on wet and dry runways to establish that the anti-skid and/or autobrakes will function compatibly and acceptably with the radial tires.
9 A high speed takeoff at the highest speed for which approval is being sought, and sufficient taxi and turn tests to assess dynamic stability should be conducted.
1 0 If the airplane is approved for unimproved runways, additional evaluations should be conducted if there is an appreciable change in the tire footprint.
1 1 Water spray characteristics should be reviewed and, if necessary, reassessed.
1 2 Tire intermix, radial/bias, should be considered based on an evaluation on a case by case basis. For example, some of these items (1-11) may not apply for a nose tire 27
intermix. Any additional considerations deemed necessary by the cognizant ACO should be addressed.
1 3 If there is a significant tire tread change from one radial tire to another radial tire, the airplane performance should be considered and if necessary reassessed.
1 4 Additional testing may be required if certification for an increase in airplane weight or kinetic energy is requested.
(5) Certification of Radial Tire Installation. The following is extracted from an FAA memorandum dated March 28, 1984, which addresses as to why is it necessary to consider items
(4)(b)1. through (4)(b)14. addressed in the preceding paragraph, and provides historical information on the reasons for certain items contained in the radial tire certification program. It also explains some of the reasons why dynamometer testing alone is not considered adequate for installation approval of replacement tires.
(a) The "Certification Program for Replacing Bias Tires with Radial Tires" dated January 12, 1984, is still the FAA recommended program. A bias ply tire to radial ply tire change is not considered minor. Dynamometer tests are not an acceptable substitute for airplane tests for tires, wheels, and brakes. This position, which is supported by FAA certification experience, is taken in AC 25-7A, chapter 4, section 4, paragraph 55.
1 For example, the FAA was requested to certificate a medium transport with three sets of tire/wheel/brake combinations for an increased gross weight. The manufacturer wished to certificate these combinations based on extrapolated data from existing lower energy data of one set, and to the higher TSO authorized energy levels of all three sets;
however, the FAA insisted a full testing program be done. All three sets failed to achieve the projected braking coefficient of friction (µ). In addition, the set that was supposed to have the greatest margin, the set designated for future growth potential, almost failed due to shearing of the brake rotor lugs. The wheels, tires and fuseplugs functioned properly.
(b) Another example was the certification of a new model large transport. The tires had received TSO authorization, but were failing at the high speed takeoffs within the TSO performance envelope. Going further back, the FAA has had similar experiences relative to fuseplugs, anti-skid brake lock-up, and so forth. These experiences confirm that the dynamometer is not a satisfactory tool in predicting performance, compatibility, fuseplug melt/no-melt, etc., for tires, wheels, and brakes. The dynamometer is even questionable when it is used for trend data, as demonstrated by the medium transport example above. It is, however, useful as background information.
(c) That the dynamometer does not accurately simulate actual conditions may be due to its inherent inability to adequately simulate the following:
1 Actual runway conditions (i.e., round versus the flat surface of the runway, surface conditions and so forth).